Gold-tin (Au--Sn) eutectic solders are commonly used in the optoelectronic and microelectronic industries for chip bonding to dies. Au--Sn solder is classified as a "hard solder" with superior mechanical and thermal properties relative to "soft" solders, such as the Pb--Sn system. Au--Sn solder can be applied in a number of ways, i.e., as Au--Sn preforms, solder paste, by sequential evaporation and sequential electrodeposition. Compared with solder preforms and pastes, evaporated solder is cleaner and provides more precise thickness and positional control. Thin film deposition technology, however, involves expensive vacuum systems.
Electroplating of Au--Sn eutectic solder is an attractive alternative in that it is a low cost process, offering the thickness and positional control of thin film techniques. Au--Sn solder layers have been produced sequentially by depositing Au first on a seed layer, followed by Sn (see for example C. Kallmayer, D. Lin, J. Kloeser, H. Oppermann, E. Zakel and H. Reichl, 1995 IEEE/CPMT International Electronics Manufacturing Technology Symposium, (1995) 20; C. Kallmayer, D. Lin, H. Oppermann, J. Kloeser, S. Werb, E. Zakel and H. Reichl, 10th European Microelectronics Conference, (1995) 440; and E. Zakel and H. Reichl, Chapter 15, in Flip-Chip Technologies, ed., J. Lau, McGraw-Hill, (1995) 415).
Commercially available Au and Sn baths are utilized from which several microns of solder can be deposited sequentially. Co-electrodeposition of Au and Sn from a single solution offers the same economic advantage of sequential plating relative to vacuum deposition techniques, as well as the prospect of depositing the solder in a single step without oxidation of an outer Sn layer.
The technology for Au and Sn plating is quite well developed and will be briefly reviewed here.
Au Electrodeposition
Electrodeposition of soft Au on electronic devices and componenets is generally performed using a bath containing cyanoaurate (I) ions, because Au cyanide complexes have the highest stability coefficients. Free cyanide ions generated as a result of the Au deposition process attack the interface between the resist film and substrate, lifting the resist and depositing extraneous Au under the resist. Because of this incompatibility, work has focused on developing non-cyanide baths.
Au(I) sulphite complexes have better compatibility towards positive resists and the added benefit of improved throwing power and deposit thickness uniformity compared with cyanide baths. In addition, deposits from Au sulphite solutions are bright, hard and ductile. The Au(I) sulphite complex is subject to a disproportionation reaction, however, forming Au(III) and metallic Au, which causes the bath to decompose spontaneously on standing. EQU 3[Au(SO.sub.3).sub.2 ].sup.3- =2 Au+[Au(SO.sub.3).sub.4 ].sup.5- +2 SO.sub.3.sup.2-
To prevent decomposition, a suitable stabilizing additive is needed.
The first commercial sulphite Au plating solutions were developed in the early to mid 1960s. The sulphite ion is itself in equilibrium with sulphur dioxide according to EQU SO.sub.3.sup.2- +H.sub.2 O=SO.sub.2 (g)+2 OH.sup.-
Because the above reaction forms hydroxyl ions, the equilibriun is pH-dependent. Most commercial solutions operate in the alkaline pH range, i.e., at pH values above9.5. When Au is plated out of solution at alkaline pH, the excess sulphite remains and can be oxidized to sulphate at the anode.
There have been several attempts to reduce the operating pH to below neutral for applications involving alkaline-developable photoresists (see for example A. Gemmler, W. Keller, H. Richter and K. Ruess, Plating and Surface Finishing, 81 (1994) 52; R. J. Morrissey and R. I. Cranston, U.S. Pat. No. 5,277,790, Jan. 11, 1994; R. J. Morrissey, Plating and Surface Finishing, 80 (1993) 75; and T. Osaka, A. Kodera, T. Misato, T. Homma, Y. Okinada and O. Yoshioka, J. Electrochem. Soc., 144 (1997) 3462).
The addition of organic polyamines, such as ethylenediamine, can be used to lower the pH to acidic values, allowing controlled evolution of sulphur dioxide to remove a portion of the excess sulphite (U.S. Pat. No. 5,277,790; R. J. Morrissey, Plating and Surface Finishing (above); and A. Meyer, S. Losi and F. Zuntini, Proc. Fachtagung. Galvanotachnik, Leipzig (1970), Swiss Patent 506,828 (1969)).
The possibility of electroplating soft Au from a non-cyanide bath containing both thiosulphate and sulphite as complexing agents has been explored (see for example T. Osaka, A. Kodera, T. Misato, T. Homma, Y. Okinada and O. Yoshioka, J. Electrochem. Soc., 144 (1997) 3462; T. Inoue, S. Ando, H, Okudaira, J. Ushio, A. Tomizawa, H. Takehara, T. Shimazaki, H. Yamamoto and H. Yokono, Proceedings of IEEE 45th Electronic Components and Technology Conference, May 21-24, 1995; and M. Kato, Y. Yazawa and Y. Okinaka, International Technical Conference Proceedings, American Electroplaters and Surface Finishers Society, (1995) 813).
The bath reported by Osaka et al. operates at a pH of 6.0 and a temperature of 60.degree. C. The bath is reported to be stable, although no specific stability data has been given. Three different Au complexes can exist in this system. EQU Au.sup.+ +2 SO.sub.3.sup.2- =[Au(SO.sub.3).sub.2 ].sup.3- .beta.=10.sup.10 EQU Au.sup.+ +2 S.sub.2 O.sub.3.sup.2- =[Au(S.sub.2 O.sub.3).sub.2 ].sup.3- .beta.=10.sup.26 EQU Au.sup.+ +SO.sub.3.sup.2- +S.sub.2 O.sub.3.sup.2- =[Au(SO.sub.3)(S.sub.2 O.sub.3)].sup.3- .beta.=unknown
.beta. is the stability coefficient for the complex. Thallium(I) ions have been added in the form of Tl.sub.2 SO.sub.4 as a grain refiner to improve the surface morphology of the deposit.
Phosphates, carbonates, acetates and citrates are commonly used as buffering and conducting agents for Au plating baths. In alkaline Au sulphite baths, metals such as Cd, Ti, Mo, W, Pb, Zn, Fe, In, Ni, Co, Sn, Cu, Mn and V in various concentrations are used as brightening additives, while Sb, As, Se and Te semi-metals are also used.
Sn Electrodeposition
There are 2 types of Sn plating solutions: alkaline and acidic (see A.C. Tan, Chapters 8-10, "Tin and Solder Plating in the Semiconductor Industry", Chapman and Hall (1993)).
Alkaline solutions are based on sodium or potassium stannate. Hydrogen peroxide or sodium perborate is used to oxidize any stannite (bivalent Sn) to the stannate form. Alkaline baths are superior to acid baths in throwing power.
Acidic plating baths contain Sn in the bivalent form, using metal salts that are sulphates, fluoroborates and fluorosilicates. Electrodeposition of Sn from a stannous Sn solution has the obvious advantage of consuming less electricity (half the amount at 100% efficiency) compared with a stannate bath. The problems with acidic baths include poor throwing power and solution instability, with basic tin compounds precipitating on standing. Various additives, including gelatin, glue, cresol sulphonic acid and aromatic hydroxyl compounds, have been used to improve plating quality. When an acidic bath ages, the bath may change colour to darker yellow and may also become turbid. The actual chemistry of this change is relatively poorly understood, but is attributed to the formation of stannic compounds when stannous Sn salt is oxidized to stannic Sn in the presence of dissolved air and elevated temperature. The stannic compounds are colloidal and very difficult to remove. Oxidation of stannous Sn can be minimized by maintaining the solution temperature at 20-25.degree. C., using an airtight plating setup and adding a suitable anti-oxidant such as a phenol compound. It has been reported that oxidation of bivalent Sn can be greatly suppressed or even eliminated by adding at least 1 organic ring compound, which has a radical group such as NH.sub.2 or NO.sub.2 attached in the ortho or para position.
Au--Sn Coelectodeposition
The available information concerning the electrodeposition of Au--Sn alloys is limited. One of the problems with Au--Sn alloy plating baths is preventing the oxidation of Sn(II) to Sn(IV) which is discussed in D. R. Mason, A. Blair and P. Wilkinson, Trans. Inst. Met. Finish., 52 (1974) 143. Oxidation of Sn can be minimized by using soluble Sn anodes, however, Au is deposited on the anodes unless they are isolated by semy are isolated by semi-permeable diaphragms.
It has also been reported that Au--Sn alloys containing up to 30 at % Sn could be deposited from baths containing no free cyanide, and containing the Sn as its stannate complex formed with KOH (see E. Rau and K. Bihlimaier, Galvanische Weissgolniederschlage, Mitt. Forschungsinst. Probierants. Edelmetalle Staatl. Hoheren Fachschule Schwab. Gmund, 11 (1937) 59). Later claims concerning Au--Sn alloy plating, however, have been based on the use of alkaline and acid cyanide electrolytes, where Sn in many cases has been incorporated with the goal of obtaining brightening effects rather than producing deposits with significant amounts of Sn.
Several cyanide based systems have been reported (see T. Frey and W. Hempel, DE 4406434, (1995); W. Kuhn, W. Zilske and A.-G. Degussa, Ger. DE 4,406,434, Aug. 10, 1995: N. Kubota, T. Horikoshi and E. Sato, J. Met. Fin. Soc. Japan, 34 (1983) 37; and Y. Tanabe, N. Hasegawa and M. Odaka, J. Met. Fin. Soc. Japan, 34 (1983) 8). Frey and Hempel developed a bright Au--Sn plating bath with a pH of 3-14, comprised of potassium dicyanoaurate, soluble Sn(IV), potassium hydroxide, potassium salt of gluconic, glucaric and/or glucaronic acid, conductivity salt, piperazine and a small amount of As. The bath was used to plate small parts with an alloy containing 5-25 wt % Sn. Bright deposits were obtained for thicknesses greater than 0.1 .mu.m and the solution exhibited long term stability without the use of soluble Sn anodes. A.-G. Degussa, Ger. DE 4,406,434 teaches using potassium dicyanoaurate and tin chloride and claims a deposit composition of 8 wt % Sn and thickness of 5 .mu.m.
Au--Sn codeposition from a cyanide system using pyrophosphate as a buffering agent was studied by Kubota et al (N. Kubota, T. Horikoshi and E. Sato, J. Met. Fin. Soc. Japan, 34 (1983) 37; and N. Kubota, T. Horikoshi and E. Sato, Plating and Surface Finishing, 71 (1984) 46). The basic formula consisted of K.sub.4 P.sub.2 O.sub.7, KAu(CN).sub.2 and SnCl.sub.2.2H.sub.2 O. The mass transfer was investigated to clarify reaction mechanisms between monovalent Au or bivalent Sn and pyrophosphate ions, by measuring conductivity, kinematic viscosity and limiting current density of the bath components. Two pyrophosphate ions were complexed with 1 stannous ion, with excess pyrophosphate acting as a supporting constituent.
Tanabe et al, referred to above, obtained various Au--Sn alloy compositions by electrodeposition from cyanide baths containing HAuCl.sub.4.4H.sub.2 O, K.sub.2 SnO.sub.3.3H.sub.2 O, KCN and KOH. Although a linear relationship was not found between the Sn content in the bath and the Sn content in the alloy formed, a relationship was found between the 2 alloys which permitted formation of alloys of desired compositions. The composition of electrodeposited Au--Sn was shifted by about 10% to the Sn side in comparison with alloys at thermal equilibrium; thus exhibiting the .xi. phase in the 25-29 at % range. AuSn, AuSn.sub.2 and AuSn.sub.4 were also electrodeposited.
Gold chloride electrolytes were used in the early days of Au plating, but today are employed almost exclusively in the electrochemical refining of Au. An extensive investigation of the cathodic behaviour of Au in chloride solutions has shown that the quality of the cathode deposit is strongly influenced by the relative amounts of Au(I) and Au(III) in the solution. The reduction of Au(III) chloride to the metal can be expected to involve the formation of Au(I) as an intermediate species. Under plating conditions, Au will be deposited from both the Au(III) and Au(I) species. Since Au(I) has a more positive plating potential (1.154 V) than Au(III) (1.002 V), a limiting current density for Au(I) will be reached first and it can be expected that the deposits will be of relatively poor quality, i.e., they tend to be bulky and porous. Gold fines will be present in the solution as a result of the following disproportionation reaction: EQU 3 AuCl.sub.2.sup.- =2 Au+AuCl.sub.4.sup.- +2 Cl.sup.-
Detailed studies of the anodic and cathodic reactions have shown that the use of low temperatures and periodic interruption of the current are major factors that can contribute to reduced Au(I) concentration.
Alkaline pH
Japanese patent JP56 136994 to Masayoshi Mashiko describes a process carried out under alkaline conditions and employing a bath composition containing gold, tin and copper and sodium sulfite or potassium sulfite was used as a stabilizer for the gold.
Acid pH
Japanese patent to S. Matsumoto and Y. Inomata, JP 61 15,992 [86 15.992], (Jan. 24, 1986) discloses a Au--Sn plating bath (pH=3-7) containing KAuCl.sub.4, SnCl.sub.2, triammonium citrate, L-ascorbic acid, NiCl.sub.2 and peptone. A 7 .mu.m Au--Sn alloy (20.+-.2 wt % Sn) layer was plated out on a 50 mm diameter Si wafer at 208 C. and a current density of 0.6 A/dm.sup.2 in 30 min using a Pt-coated non-consumable Ti anode. The stability of the bath seemed to be the weak link in this process as stability decreased dramatically when the Sn salt was added.